Al-Qaeda plotter Zacarias Moussaoui came from a broken home where his mother was repeatedly beaten, a US court in Virginia has been told.
Moussaoui has said he has no remorse over the attacks
Behavioural specialist Jan Vogelsang also said that there was a history of mental illness in Moussaoui's family.
She was speaking as an expert witness for the defence at a hearing to determine whether Moussaoui should be executed or imprisoned for life.
He has already been convicted over his role in the 11 September 2001 attacks.
'Dirty Arab' insults
Moussaoui, a French citizen of Moroccan descent, is the only person convicted in the US in connection with the attacks.
Moussaoui was in jail in Minnesota at the time, but in an earlier phase of the trial prosecutors successfully argued that he had kept federal agents from identifying and stopping some of the hijackers.
At the start of her testimony, Ms Vogelsang said that she was not trying to excuse Moussaoui's actions, but rather to help explain why he did what he did.
Ms Vogelsang said Moussaoui had been in and out of orphanages during the first six years of his life and, as a teenager in France, was rejected by the family of his long-term girlfriend as a "dirty Arab".
Ms Vogelsang also said that Moussaoui's mother, Aicha el-Wafi, was physically abused throughout her pregnancies, including six before Moussaoui was born.
Defence lawyers trying to persuade jurors to spare Moussaoui's life argue there is evidence that he is mentally ill.
They also claim he had a limited role in the 11 September plot and his execution would only fulfil his dream of martyrdom.
On Thursday Moussaoui told his sentencing trial that he had "no regret, no remorse" for the attacks on Washington and New York in which nearly 3,000 were killed.
Ms Vogelsang took the stand after a closed session with Judge Leonie Brinkema.
The defence had been expected to call "shoe-bomber" Richard Reid - serving a life sentence for attempting to blow up a trans-Atlantic flight - to the stand.
But, on Friday, Judge Brinkema annulled an order compelling Reid to appear. The jury may now hear a written statement instead.
Moussaoui has said that he and Reid were going to hijack a fifth plane on 11 September 2001 and fly it into the White House.
Lawyers for Moussaoui have suggested he fabricated the story about a fifth plane to sabotage his defence and that he is seeking to inflate his role in history.
The judge has said jurors could be sent out for deliberations this week.
The prosecution wrapped up its case on Wednesday, after presenting the jury with emotional evidence from the 11 September attacks.
A number of victims' relatives took the stand to describe how the attacks had affected their lives. The jury also heard a chilling cockpit recording from inside one of the four hijacked US airliners.
FLIGHT PATHS OF HIJACKED AIRCRAFT ON 11 SEPTEMBER 2001
American Airlines Flight 11: Boston-Los Angeles (AA 11)
0759 Flight takes off
0814 Last routine radio communication; likely takeover
0846 Fighter jets scrambled to find aircraft
0846 Flight crashes into 1 World Trade Center (north tower)
United Airlines Flight 175: Boston-Los Angeles (UA 175)
0814 Flight takes off
0842 Last radio communication
0842-0846 Likely takeover by hijackers
0903 Flight crashes into 2 World Trade Center (south tower)
American Airlines Flight 77: Washington DC-Los Angeles (AA 77)
0820 Flight takes off
0851 Last routine radio communication
0851-0854 Likely takeover by hijackers
0854 Flight makes unauthorised turn to south
0937 Flight crashes into Pentagon
United Airlines Flight 93: Newark-San Francisco (UA 93)
0842 Flight takes off
0927 Last routine radio communication
0928 Likely takeover by hijackers
0957 Passenger revolt begins
1003 Flight crashes into field in Shanksville, Pennsylvania